Cross-training personnel in the cement industry – Part 3

Check the axial thrust of the kiln and the individual thrust on support rollers

While taking bearing temperatures and walking the kiln, check the kiln to see the direction of thrust. On standard kilns with thrust rollers both uphill and downhill of the thrust tyre, it is important to note where the kiln is on a daily basis. If it is one direction, is it riding on the thrust roller continuously or does it come off of the thrust roller for a percentage of the kiln rotation? This is important to know because excessive thrusting of the kiln will cause wear of the thrust rollers and contact between the gear and pinion can be reduced as a result. Furthermore, excessive thrusting of a kiln will increase the amps on the drive motor, which will essentially draw more power to operate the kiln. On kilns that have hydraulic thrust rollers only on the downhill side, it is equally important to monitor the pressure gauges on a daily basis and record the pressure. Monitoring the conditions mentioned above will be beneficial in determining if support roller adjustments are needed to control the axial thrust of the kiln. When monitoring axial thrust, make sure to check the feed rates and rpm of the kiln to determine if operating conditions are in normal parameters.

While checking the temperatures of the bearing housings and/or roller shafts, record the direction of thrust of each individual support roller. For example, is the roller shaft in the downhill direction and thrusting the kiln uphill, or is it in the uphill direction and thrusting the kiln downhill? The direction of the roller shaft can be determined by sounding end caps on some kilns, while other kilns will require opening a view port to see the direction of the shaft. The maintenance staff can demonstrate how to find the direction of thrust. The reason why checking the direction of the support roller shafts is important is to aid the maintenance staff in knowing which roller needs to be moved if there is excessive axial thrust of the kiln. Also, if there are signs of hot bearings on a support roller it is important to know the direction of thrust of the roller shaft.

Kiln shell temperatures

Many kilns today have thermal cameras mounted to monitor the kiln shell for bricking purposes. Monitoring the shell temperatures is important because it indicates what is occurring with the refractory brick but it also affects mechanical conditions of other components. The kiln will respond differently to hotter or colder shells in how it will axially thrust against the thrust roller. It also affects shell ovality and creep readings. For example, for a cooler kiln shell there will be more creep than a hotter kiln shell. If there is more creep, there will be more clearance between the tyre and the shell, which will increase the kiln shell ovality. These are conditions the maintenance and production personnel need to be aware of in their daily operations.

Weekly inspections

Weekly inspections are mostly a review of what has been happening with conditions such as the axial thrust of the kiln, the shell temperature, bearing temperatures and the general conditions of the kiln. One item that needs to be recorded on a minimum of a weekly basis is the creep of the tyres. Creep is the relative movement of the tyre versus the kiln shell. Because the tyre ID is larger than the diameter of the kiln shell (and support pads), the tyre does not migrate the same distance circumferentially as the shell. This difference in movement is referred to as tyre creep.

Another area to check on a weekly basis is the condition of the tyres, support rollers and retaining blocks/stop blocks on each pier of the kiln. Wear patterns will sometimes develop on the surfaces of tyres and support rollers and can be an early warning sign of possible mechanical conditions on the kiln. Dark colouration across the face of a support roller is an indication of excessive thrust of this support roller. When a support roller has very little individual thrust as a result of how it is adjusted, it will normally have a bright shine to the face of the roller. When a roller has a very dark face across the wearing surface, it is an indication of excessive thrust on this roller.

One of the most common wear problems on the majority of kilns is the thrusting of the tyre against the retaining/stop blocks holding the tyre in place. This condition is the result of misalignment present between the shell and the tyre, the tyre and the roller, and/or the structural base of the pier top. In some cases it can be a combination of these three components. When this type of condition exists, it is important to monitor the wear rate of the retainer/stops to determine if the misalignment is excessive.

Miscellaneous

Always monitor any subtle changes that may occur to the kiln over an extended period of time. These can be simple things, such as vibration, which may be an indication of something that has changed in the drive train. Unusual patterns on the wear faces of the tyres and rollers are also an indication of mechanical changes. Oil leaking onto the surfaces of the support rollers as a result of over filling the bearing housings will cause changes in the axial thrust of the kiln. All of these little things will indicate different problems and by monitoring these conditions we will have a better handle on when to take action and when not to take action.

Conclusion

These checks listed above will be beneficial in monitoring or trending the operating condition of the kiln during the normal operating process. When one studies the effects of reliability and the impact it has on operational efficiencies, one unplanned outage can significantly change the overall efficiency of a plant. The surest and most cost effective way to avoid unplanned outages is to monitor the equipment, trending conditions with the approach of implementing risk analysis to determine if a planned outage for a short duration outweighs the risk of an unplanned outage with potentially much longer production losses. However, to even get to the position of performing educated risk analysis, it is critical to implement an inspection programme. In today’s environment with a shortage of skilled labour to staff maintenance positions, cross-training of non-maintenance personnel is a means of achieving goals without further burdening a depleted maintenance staff.